The present invention relates generally to the field of laser diodes, and more particularly to transfer of short-wavelength nitride-based laser diodes from transparent substrate materials onto dissimilar substrates.
Short-wavelength nitride based laser diodes provide smaller spot size and a better depth of focus than red and infrared (IR) laser diodes for laser printing operations and other applications. Single-spot nitride laser diodes have applications in areas such as optical storage.
Laser diode arrays are desirable for application to high-speed laser printing. Printing at high speeds and at high resolution requires laser arrays due to the fundamental limits of polygon rotation speed, laser turn-on times and laser power. Laser diode arrays have previously been employed using red and infrared laser diode structures. Dual-spot red lasers and quad-spot infrared lasers have been used for laser printers.
Laser diodes based on higher bandgap semiconductor alloys such as AlGalnN have been developed. Excellent semiconductor laser characteristics have been established in the near-UV to violet spectrum, principally by Nichia Chemical Company of Japan. See for example, S. Nakamura et al., xe2x80x9cCW Operation of InGaN/GaN/AlGaN-based laser diodes grown on GaN substratesxe2x80x9d, Applied Physics Letters, Vol. 72(16), 2014 (1998) and S. Nakamura and G. Fasol, xe2x80x9cThe Blue Laser Diode-GaN based Light Emitters and Lasersxe2x80x9d, (Springer-Verlag, 1997), A. Kuramata et al., xe2x80x9cRoom-temperature CW operation of InGaN Laser Diodes with Vertical Conducting Structure on SiC Substratexe2x80x9d, Japanese Journal of Applied Physics, Vol. 37, L1373 (1998) all of which are incorporated by reference in their entirety.
Extension of dual-spot lasers or quad-spot lasers to shorter wavelengths enables printing at higher resolution. The architecture for short wavelength laser diodes has needed to be different because of the misalignment of the crystallographic orientation between the sapphire and the GaN epitaxial layer. Therefore mirror facets formed by cleaving on laser diodes with sapphire substrates exhibit increased surface roughness and reduced reflectivity. The growth on sapphire substrates has also required that the p-electrode and n-electrode have to be formed on the same surface, typically with a lateral n-contact in contrast to laser diodes on conductive substrates which have a backside n-contact.
A group from the University of California has developed a technique for separation of GaN films from sapphire substrates using an UV-excimer laser. The University of California technique uses an ultraviolet excimer laser to decompose a thin portion of the GaN layer at the interface with the sapphire substrate. By proper adjustment of the excimer laser flux, the interfacial GaN is decomposed into Ga and N with minimal damage. Subsequently, the GaN film is removed by gentle heating of the remaining Ga metal which has a melting point of 30xc2x0 C. at the film-substrate interface. See W. S. Wong et al., xe2x80x9cDamage-free separation of GaN thin films from sapphire substratesxe2x80x9d, Applied Physics Letters, Vol. 72, 599 (1998) which is incorporated by reference in its entirety.
Architectures using insulating substrates allow the economical construction of nitride based laser diodes and laser diode arrays. Currently, most advanced nitride based single laser structures are grown on insulating sapphire (Al2O3) substrates which are optically transparent. The use of optically transparent insulating substrates for laser diode arrays presents a special problem in providing electrical contacts for the laser diodes. In contrast to the situation where conducting substrates are used, insulating substrates cannot provide a common backside contact for all laser diodes in an array. Hence, providing electrical contacts to laser diode arrays on insulating substrates has required the use of special architectures.
In an embodiment in accordance with the invention, removal of the optically transparent insulating substrate after growth of the laser diode or laser diode array structures simplifies providing electrical contacts to the laser diode or laser diode arrays and avoids special architectures while allowing a superior heat sink to be attached to the laser diode or laser diode arrays. The laser diode or laser diode array may be attached to a thermally conductive wafer or directly mounted onto a heatsink after substrate removal by soldering, thermo-compression bonding or other means well known to those of ordinary skill in the art. Removal of the optically transparent insulating substrate requires the attachment of a support substrate as an intermediate step. Subsequent transfer of the laser diode or laser diode array after removal of the support substrate requires a secondary support to provide mechanical rigidity to the laser diode or laser diode array. Adding a thermally conductive substrate to the laser diode or laser diode array before removal of the insulating substrate allows positioning of the thermally conductive substrate on the side of the laser diode or laser diode array closer to the laser active region for more effective heat sinking than if the laser diode or laser diode array is attached to the thermally conductive substrate after removal of the insulating substrate. This is particularly important in the case of independently addressable laser diode arrays used in high-resolution and high-speed printing. Any cross-talk between laser diodes in an array adversely effects the performance of the printing system and is to be avoided. Thermal cross-talk is a major component in the case of nitride based lasers grown on sapphire because of the comparatively poor thermal conductivity of the sapphire substrate. Removal of the sapphire substrate greatly reduces the thermal impedance and consequently any thermal cross-talk is also reduced.
The nitride laser membrane may be cleaved to create parallel mirror facets before attachment to the new host substrate. The nitride laser membrane may also be aligned during the attachment and transfer process with a new crystallographically oriented host substrate and cleaved to form mirror facets for the laser diode or laser diode array. Cleaved rather than etched mirror facets result in perfectly parallel, vertical, and smooth mirrors which are critical for properly optimized laser operation. Also a free-standing nitride laser membrane may be cleaved directly without attachment to any substrate to form high-quality mirror facets.